Damping is the absorption of energy, such as vibrational or sound energy, by a material in contact with the source of that energy. It is desirable to damp or mitigate the transmission of vibrational energy from a number of sources such as motors, engines, and other power sources.
It is known to employ viscoelastic materials for damping applications. In general, energy, such as vibrational energy, is absorbed by the viscoelastic material and the energy is subsequently converted into heat rather than being transferred to the environment as vibrational energy. Ideally, the viscoelastic materials employed for dampening are useful and effective over a wide range of temperatures and frequencies.
The viscoelastic nature of materials can be mathematically represented by the formula G*=G'+iG" where G* is the complex shear modulus, G' is the elastic or storage modulus, G" is the viscous or loss modulus and i=-1+L . The effectiveness of an viscoelastic material for damping purposes can be quantified by measuring its viscoelastic response to a periodic stress or strain. Results of dynamic mechanical tests are generally given in terms of elastic or storage modulus, G', and viscous or loss modulus, G". The loss modulus G" is directly related to the amount of mechanical energy that is converted to heat, or in other words, damping.
The ratio of the loss modulus G", to the elastic modulus, G', is denoted by ##EQU1##
which is a parameter that quantifies the ability of a material to dissipate mechanical energy into heat versus the purely elastic storage of mechanical motion during one cycle of oscillatory movement. The measurement of tan (.delta.) can be made by a dynamic analyzer, and may be made by a sweep of frequencies at a fixed temperature, then repeating that sweep of frequencies at several other temperatures, followed by the development of a master curve of tan (.delta.) versus frequency by curve alignment. An alternate method is to measure tan (.delta.) at constant frequency over a temperature range.
In common practices, the tan (.delta.) of a material is usually adjusted or broadened by taking advantage of the glass transition temperature of several materials within a temperature range. U.S. Pat. No. 5,494,981 teaches a composition that comprises resins that are cured in sequential fashion by using a single catalyst. The catalyst in this invention is a Bronsted acid, which activates an epoxy resin component and then activates cyanate trimerization into poly(triazines). The composition provides a glass transition damping peak around 100.degree. C. and is understood to be heat stable over a temperature range of about 0.degree. to at least 300.degree. C.
U.S. Pat. No. 5,008,324 teaches a multi-phase thermoplastic elastomeric damping additive, and compositions containing the multi-phase thermoplastic elastomeric polymer. For example, a composition used for damping can include a soft crosslinked elastomeric binder containing microscopically discrete segments of the multi-phase thermoplastic elastomeric polymer. The multi-phase thermoplastic elastomeric polymer or damping additives have at least two polymeric phases including an initial linear or lightly linked polymeric phase and a second polymeric phase in the form of discrete domains dispersed within the initial polymeric phase.
U.S. Pat. No. 5,225,498 teaches a damping material that includes an interpenetrating polymer network having a soft polymer component made of polyurethane and a hard polymer component made of a vinyl ester polymer. The polyurethane and the vinyl ester polymer are polymerized in the presence of one another and cured at room temperature. The interpenetrating polymer network is taught to have an acoustic damping factor in excess of 0.2 over a temperature range of from about 15 to about 85.degree. C., with a glass transition damping peak at about 55.degree. C.
U.S. Pat. No. 5,670,006 teaches a composition for vibration damping, which includes an acrylate-containing thermoset resin that incorporates an interpenetrating network of polymerized epoxy monomer and polymerized acrylate monomers. The epoxy-acrylate thermoset resin is taught to have a glass transition temperature in the range of about -2.degree. C. to about 200.degree. C. at 1 Hz.
Although numerous compositions are known for damping, there is a need for improved damping compositions that exhibit a high degree of damping over a wide range of temperatures and frequencies without involving glass transition peaks. Enhancing hysteresis (tan (.delta.)) by using super position of glass transition peaks is not desirable because the modulus of the material drops dramatically at or about the glass transition temperature.